1. Field of the Invention
The present invention relates to the optical testing of light-emitting devices and displays. More specifically, the invention relates to a test apparatus which may be used, in conjunction with conventional automatic test equipment, to optically test a variety of light-emitting devices.
2. Discussion of the Prior Art
As a result of the advent of automatic test equipment (ATE), the testing of the electrical functions of complex devices has become highly automated. For example, a printed circuit board which contains a variety of discrete components, integrated circuits and other devices may be subjected to comprehensive electrical testing by means of a "bed of nails" test fixture. Printed circuit boards, however, frequently include light-emitting components such as light-emitting diodes, seven-segment displays and the like, which must be tested both electrically and optically. Optical testing may determine whether a component is actually emitting light when it should be, whether the emission exceeds a minimum acceptable intensity, whether the intensities among several components are sufficiently uniform, etc.
One technique of optically testing light-emitting devices involves a machine vision system in which a television camera is aimed at a light-emitting device under test (DUT). The image produced by the camera provides information on emitter intensity and background intensity on a pixel-by-pixel basis. An analog-to-digital conversion is then performed and the equivalent binary values are stored in memory.
The disadvantages of this type of vision system are numerous. First, the output signals provided by the vision system are typically not electrically compatible with conventional ATE inputs, thus preventing a direct connection of the vision system with an ATE host. As a result, the optical testing of the DUT cannot be directly combined with the electrical testing into a common automatic test program.
Second, for certain types of optical testing, the vision system produces large amounts of data which are not useful. For example, assume that the object of the test is to determine simply whether a particular light-emitting device is illuminated at a particular time. Assume also that the DUT is a printed circuit board which contains several widely spaced light-emitting diodes and that the viewing angle of the camera is wide enough to cover the entire surface of the circuit board. Under these conditions, most of the pixels in the image produced by the camera will not be of the diodes themselves and are therefore not of interest. Moreover, most of the corresponding binary data is irrelevant and a data reduction process must be performed, which typically requires complex image processing software.
Third, the camera provides a fixed, uniform spatial resolution across its entire field of view. This may represent a significant disadvantage in cases where the DUT contains several widely spaced clusters of closely spaced light-emitting devices. In order to resolve two adjacent light-emitting devices while also monitoring the entire DUT, the vision system may require additional cameras, thereby increasing the cost and complexity of the system.
Finally, the cost of the vision system is relatively high due to the necessity of the camera(s), the large memory required to store the data and the image processing software.